1. Field of the Invention
The present invention relates to chemical vapor deposition (referred to herein as “CVD”), and in particular, it relates to a CVD apparatus suitable for depositing films on large-scale flat panel substrates.
2. Description of Related Art
Known methods for the production of large-scale liquid crystal displays include methods that use high-temperature polysilicon TFTs (thin film transistors) and methods that use low-temperature TFTs. In liquid crystal display production methods that use low-temperature polysilicon TFTs, there is no need to use expensive substrates such as quartz because all the processes can be performed at a low temperature (e.g., 400° C. or less).
It is also possible to achieve cost reductions by increasing the production yield if the drive circuits for driving the devices in the liquid crystal displays, and like devices, are built into the substrate at the same time. Since this also has the effect of improving the TFT device characteristics, it makes it possible to increase the degree of detail and achieve a larger aperture ratio. Consequently, painstaking research is being undertaken with a view to achieving improved performance, and the volume of production itself is also increasing.
In the production of liquid crystal displays using low-temperature polysilicon TFTs, plasma CVD is used for the low-temperature deposition of polysilicon oxide films, which are suitable for use as gate insulation films.
For such applications, a CVD apparatus proposed in a previous patent application (U.S. patent application Ser. No. 09/435,625, the subject matter of which is hereby incorporated herein by reference) involves producing a plasma inside a vacuum enclosure to generate excited active species (referred to herein as “radicals”) and using these radicals and a precursor gas to deposit a film on a substrate. Specifically, this apparatus uses a technique whereby a dividing plate, having a plurality of holes through which the radicals pass, is used to separate the interior of the vacuum enclosure into a plasma discharge space and a film deposition space. Radicals are generated from the plasma by introducing a gas into the plasma discharge space, and these radicals are introduced to the film deposition space through the plurality of holes in the above-mentioned dividing plate. Meanwhile, a precursor gas is directly introduced into the film deposition space from outside the vacuum enclosure without coming into contact with the above-mentioned plasma or radicals. The precursor gas is allowed to react with the above-mentioned radicals introduced into the film deposition space, whereby a film is deposited on a substrate (e.g., on a glass substrate measuring 370 mm×470 mm) situated in the film deposition space.
An example of a thin-film deposition apparatus used for plasma CVD that uses a dividing plate 24 to separate the interior of the vacuum enclosure into a plasma discharge space and a film deposition space is described using FIG. 1(a) and (b). FIG. 1(a) is a cross-sectional view of a conventional dividing plate 24, and FIG. 1(b) is a plan view of the interior as seen from line X-X in FIG. 1(a).
The dividing plate 24 consists of a three-plate laminated structure where an intermediate diffusion plate 2 is sandwiched between an upper plate 1 and a gas discharge plate 3 on the film deposition side, and these three plates are fixed at their outer perimeter. The fixing at the outer perimeter of these three plates (upper plate 1, intermediate diffusion plate 2, and gas discharge plate 3 on the film deposition side) can, for example, be achieved by using screw fixing members 9 as shown in the figure, or by welding or the like (not illustrated).
The dividing plate 24 consisting of three plates laminated and fixed in this way has spaces provided in the interior thereof, i.e., precursor gas primary diffusion spaces 4 and precursor gas secondary diffusion spaces 5, and these internal spaces 4, 5 are connected together by intermediate gas distribution holes 6. A precursor gas, which is fed from outside into the vacuum enclosure of the thin-film deposition apparatus, is uniformly diffused as it passes through, in sequential order, the precursor gas primary diffusion spaces 4, the intermediate gas distribution holes 6, and the precursor gas secondary diffusion spaces 5, and is then guided from the precursor gas discharge holes 7 into the film deposition process chamber (the lower part in FIG. 1(a)).
Meanwhile, radical transit holes 8 are provided in the parts where there are no spaces inside the dividing plate 24, and the radicals produced in the plasma discharge space (i.e., above the dividing plate 24) pass through these radical transit holes 8 and are guided into the film deposition process space below the dividing plate 24.